The present invention relates to data reproducing devices, and more particularly to a data reproducing device for reproducing data from a recording medium on which video data and audio data have been recorded in a multiplex mode.
Japanese patent application No. 45780/1983 discloses a system in which video data and audio data are recorded in a multiplex mode, and audio signals of wide dynamic range and high quality can be obtained. In this system, frequency modulation signals obtained by frequency-modulating a video signal and an audio signal are recorded on a recording medium. In addition, a pulse train signal obtained by digitizing an audio signal according to a predetermined digital modulation system is superposed on the frequency modulation signals recorded on the recording medium.
In this system, the audio signals are of two channels, and 2.3 MHz and 2.8 MHz audio carriers are frequency-modulated with two audio channel signals. The video signal is frequency converted so that it has a sync peak of 7.6 MHz, a pedestal level of 8.1 MHz, and a white peak of 9.3 MHz. The audio signal is converted into a pulse train signal by being digitized according to a modulation system such as a pulse code modulation (PCM) system. The pulse train signal is processed into a signal suitable for recording, for instance, by an EFM (eight-to-fourteen modulation) system. The .frequency spectrum includes the frequency components of a pulse train having widths of 3T to 11T, where T is the bit period of a PCM signal, a pulse having a width of 3T is of about 720 kHz, and a pulse having a width of 11T is of about 200 kHz. Such a pulse train signal is superposed on the video main carrier with a level of about 1/10 or less, and is subjected to slice modulation near the zero crossings, as a result of which a pulse-width-modulated signal is obtained as a recording signal.
FIG. 1 shows the frequency spectrum of an RF (high frequency) signal which is obtained from a recording medium on which video data and audio data have been recorded according to the above-described recording system. In FIG. 1, reference character A designates the spectrum of a digitized audio signal; B, the spectrum of an audio FM signal; and C, the spectrum of a video FM signal. As the dynamic range of the digitized audio signal can be 90 dB or more, the tone quality is remarkably better than in the case of recording and reproducing audio data using a frequency modulation system.
In order to reproduce the audio signal thus digitized and recorded, it has been proposed to provide a PCM decoder adapted to decode and reproduce the digitized audio signal for a conventional data reproducing device which carries out only the reproduction of frequency-modulated and recorded video signals and audio signals. The PCM decoder has a buffer memory which stores PCM data successively. In the PCM decoder, the PCM data is read out of the buffer memory with the same period as the sampling period and decoded to reproduce audio signals. If, in the case where the data reproducing device is provided with such a PCM decoder, the PCM data reading speed, i.e., the speed of movement of the data detecting point relative to the recording medium, is not accurately controlled, overflow may take place in the buffer memory in the PCM decoder to which the PCM data are supplied.
With a data recording device provided with such a PCM decoder, it is necessary to selectively supply through a common output terminal to subsequent audio equipment either a reproduction signal obtained by reproducing an audio signal which was recorded in frequency-modulated form or an audio signal recorded in digitized form. In this case, if the switching operation for the selection is not automatically carried out, a complex manual switching operation is required, resulting in further drawbacks of the device.